Shaft bushings in plates (e.g. walls, guard-plates or the like) can be sealed in various manners in order to prevent contaminations or liquid from passing along the shaft. In cases of axially movable, reciprocating shafts flexible bellows of rubber or plastics are generally used, which are connected in a liquid-tight manner on the one hand to the shaft, on the other hand to the plate along the edge of the hole through which the shaft extends. In cases of shafts with large axial movements seals of very high flexibility are required, not only in the longitudinal direction of the shaft but also radially, since shafts of this type during maneuvering frequently move a little in a lateral direction. Therefore, in this type of shaft bushing rubber bellows and the like occasionally cannot be used.
Within certain technical fields, e.g. in machines for the packaging of liquid foodstuffs, the packing machine is divided into a "wet" and a "dry" zone. In the wet zone filling of the packing containers takes place along with a part of the forming work and closing of the packing containers. It must be possible to clean the wet zone with strong washing agents and to flush it with water and possibly steam. The dry zone comprises the driving and lubricating system of the machine which must be completely separated from the wet zone. Between the two zones, which are separated from one another by means of a frame plate of stainless steel, run shafts with axial as well as rotational movement. The sealing of these shafts has to be done very carefully, on the one hand because of the handling of delicate food substances (milk etc.) which on no account must be allowed to take up the flavor of, or be contaminated by, the driving system of the machine, on the other hand because of the use of harsh cleaning and sterilizing agents which must not be permitted to enter into the dry zone of the machine.